Due to fundamental limitations of material purification methods, materials are never 100% pure, which by definition induces defects in crystal structure. Substitutional defects result from an impurity present at a lattice position. For the substitutional type, solute or impurity atoms replace or substitute for the host atoms. Several features of the solute and solvent atoms determine the degree to which the former dissolves in the latter. These are expressed as the Hume–Rothery rules. According to these rules, substitutional solid solutions may form if the solute and solvent have:
- Similar atomic radii (15% or less difference)
- Same crystal structure
- Similar electronegativities
- Similar valency a solid solution mixes with others to form a new solution
A solid solution is a uniform mixture of two crystalline solids that share a common crystal lattice. Solid solutions often consist of two or more types of atoms or molecules that share a crystal lattice, as in certain metal alloys. Solvent is the element or compound that is present in the greatest amount. Solute is used to denote an element or compound present in a minor concentration. The solute may incorporate into the solvent crystal lattice substitutionally, by replacing a solvent particle in the lattice, or interstitially, by fitting into the space between solvent particles. Both of these types of solid solution affect the properties of the material by distorting the crystal lattice and disrupting the physical and electrical homogeneity of the solvent material.